Method and apparatus for improving recovery of plutonium-gallium alloys by electrorefining



J. A. LEARY ET AL 3,417,002

ING RECOVERY OF Dec. 17, 1968 METHOD AND APPARATUS FOR IMPROVPLUTONIUM-GALLIUM ALLOYS BY ELECTROREFINING Filed June 15. 1965 2Sheets-Sheet 1 POWER SUPPLY n N U c m w mm A III O 0 ii: 0 O

I I I I I I I litllnilflil Jill! I!!! J. E Baa/um United States Patent()7 METHOD AND APPARATUS FOR IMPROVING RECOVERY OF PLUTONIUMGALLIUM AL-LOYS BY ELECTROREFlNlNG Joseph A. Leary, Lawrence J. Mullins, Jr., andJohn F. Buchen. Los Alamos, N. Mex., assignors to the United States ofAmerica as represented by the United States Atornlc Energy CommissionFiled June 15, 1965, Ser. No. 464.250 5 Claims. (Cl. 204-) ABSTRACT OFTHE DISCLOSURE A device and method for recovering plutonium fromplutonium-gallium alloys by an electro-refining process in which thesaid alloy is the anode of the electrorefining cell and the E.M.F. ofthe said cell reaction between the cathode and anode supplies thevoltage to drive a control circuit, and said method consists of addingto the anode iron, cobalt, or nickel in an amount equal to about weightpercent of the gallium present.

This invention relates to an apparatus and method for extending theamount of plutonium which can be recovered in an eiectrorefining processand more particularly to an apparatus and method in which the source ofthe plutonium is a plutonium-gallium alloy, said alloy being the anodeof the eiectrorefining cell and said process consisting of adding asmall amount of iron, cobalt, or nickel in powdered form to the liquidmetal anode before initiating the electrorefining operation.

Normally, alloys of plutonium and gallium contain about one percent byweight of gallium in order to stabilize the delta phase of plutonium.This composition is a uniform liquid metal solution at 750 C. which isthe normal operating temperature of the electrorefining operalion. Theliquid metal plutonium-gallium alloy serves as the anode of theelectrorefining cell, and the plutonium is dissolved selectivley out ofthe solution thereby concentrating the gallium in the anodic solution.Eventually, this liquid metal anode composition reaches a point where noliquid remains and the anode becomes completely polarized. At 750' C.this point is reached when the atomic ratio (R) of gallium to plutoniumreaches the value of about 0.30. The electrorelining operation must beterminated at this point and yet gross amounts of plutonium still remainin the polarized anode. The invention herein disclosed relates to amethod for extending the amount of plutonium that can be dissolvedanodically before anode polarization becomes serious; that is, the ratio(R) of gallium to plutonium is greater than 0.30. This invention is alsooonoemed with an electronic system or apparatus for the electrorefiningof plutonium from its alloys and in particular plutonium-gallium alloys.This system insures the produ :tion of high purity plutonium metal inoptimum yield by permitting the electrorefinlng process to continue onlyas long as high purity metal is being produced safely. Without thiscontrol system, previous experience has shown that unsafe quantities ofpynophoric alkali metals could be generated within 3,417,002 PatentedDec. 17, 1968 reviewing the previous development history. in early workon the electrorefining of plutonium alloys, much difficulty wasexperienced on knowing when to terminate the electrorefining process.When the electrorefining run was terminated prematurely the process wasinefficient. If the operation was carried beyond a certain degree ofdepiction of the liquid plutonium anode an unsatisfactory product wasobtained, even though the concentration of plutonium ions in the moltensalt electrolyte remained essentially unchanged. it was thereforenecessary that some property of the liquid metal plutonium anode must bemonitored and correlated with the impurity of the product obtained bythe electrorcfining process. The inventors have discovered that the backelectromotive forces measured only on an open circuit between the anodeand the cathode of the cell was essentially independent of theconcentration of plutonium ions present in the molten salt electrolyte.However, when this same back E.M.F. increased from the very low normaloperating value of approximately 0.1 volt to the value of 0.25 volt, itwas essential to terminate the process in order to insure both maximumyield and purity. in the apparatus of this invention the back E.M.F.monitors the condition of liquid plutonium metal at the anode and doesnot monitor the concentration of plutonium ions in the molten saltelectrolyte, and therefore, does not use the concentration of the moltensalt electrolyte as the control signal which is the case inelectrorefining cells of the prior art. This method of using the backE.M.F. is unique to our electrorefining system which employs a stirredliquid metal anode and it is also noteworthy that in the process of thisinvention the impure feed material is a plutonium metal alloy and notplutonium chloride. This difference necessitates differences in the celldesign and the use of the back E.F.F. method to control same.

Accordingly, there is provided by the present invention an automaticcontrol system for the electrorefining of plutonium alloys and inparticular plutonium-gallium alloys. The system consists basically oftwo components, an external electronic control system and the additionof a certain amount of a metal added to the anode of the system selectedfrom the class consisting of iron, cobalt, nickel, ruthenium, rhodium,palladium, osmium and platinum. For other objects and advantages of theinvention and for other modifications thereof, reference is now to behad to the following detailed description taken in conjunction with theaccompanying drawings, in which:

FlGURE 1 shows a vertical cross-sectional view of the electrorefiningcell; and FIGURE 2 diagrammatically illustrates a wiring diagramexemplary of the preferred form of the invention.

Turning now to FIGURE 1, it will be seen that the cell consistsessentially of an outer ceramic vessel 2 com taining a fused 'saltelectrolyte band an inner shorter ceramic vessel 1 separated from outervessel 2 by an annulus filled with electrolyte 9. Inner vessel 1 servesas the container for the impure anode 7. Depending upon exteriorstructure (not shown) and extending into anode 7 is a metallic rod 4 anda ceramic stirrer 3. This rod 4 is sheathed by a ceramic tube 6 andserves as the anode lead 4. The anode lead 4 is connected to the powersupply through Q, an ampere-hour meter 11, and in the same circuit is avoltmeter 12 that is a back E.M.F. indicator. The stirrer can be rotatedby means well known in the are with the upper stirrer circulating theelectrol to and thus preventing the isolation of a plutonium eplctedlayer adjacent to the cathode 8 while the lower stirrer promotes contactbetween plutonium anode 7 and electrolyte 9. The plutonium which platesout on the cathode drips to the bottom of vessel 2 and collects in apool 8. Cathode 8 is mounted inside of outer vessel 2 partiallyextending therefrom and largely immersed in the electrolyte. Cathode 5is connected to a power supply through switch 13 and ammetcr 10. Switch13 permits interrupting the power supply to the cell during the periodthe cell potential is being measured.

Electrolyte 9 is a combination of fused salts having a relatively lowmelting point and must include a plutonium salt. The inventors havefound that a chloride or chloride-fluoride electrolyte consisting of 10weight percent plutonium trichloride -or plutonium trifiuoride orplutonium tetrafiuoride, weight percent potassium chloride, and 40weight percent sodium chloride is a useful electrolyte solution. Thiscell is essentially the same as appears in US. Patent No. 3,098,028 andthis patent is hereby expressly incorporated as part of thisapplication.

In FIGURE 2 we have a diagram of the electrorefin ing electronic circuitwhich is divided into circuit A and circuit B. The combination of celland electronic circuit can accomplish what neither can do alone in thatthe cell of FIGURE 1 has a unique feature that its back E.M.F. isnegligible when the cell is functioning properly. The back E.M.F. risesrapidly when either the plutonium value in the alloy is depleted or amechanical failure such as a stirrer stoppage occurs. The electronicsystem measures the back E.M.F. of the cell once each hour or at anydesired preset time interval and if the said E.M.F. is within allowablelimits the circuit permits the eleetrorefining process to continue;otherwise, the eleetrorefining cell is automatically disconnected fromthe power supply thus terminating the process. The following is adescription of circuit A as used in a preferred embodiment of thisinvention:

CIRCUIT A The control power for circuit A is derived from the 115 volthertz AC line through S101 power," and the circuit protection fuse F101.The coil of relay K102 of circuit A with its contacts K102A and K102i ofcircuit B is energized once each hour. Relay K102 is energizied onceeach hour or at any preferred time interval for a period ofapproximately 2 to 4 seconds. This energizing of the relay isaccomplished by a motor driven cam switch composed of time controlTMR101, its contacts TMRIOIA. and the combination of rectifier CR101.capacitor C101, resistor R101. relay coil K101 and rectifier CR102. S102"test" is connected in parallel with the contacts TMR101A to provide formanually energizing the sampling system at any time between hourlysampling intervals controlled by the timer TMR101 and its contacts. Tokeep the dwell time of such a switch to about 2 to 4 seconds is thepurpose of the combination of CR101. C101. R101, K101, and CR102. Whenthe contacts TMR101A are closed during the sampling interval they supplycurrent to rectifier CR101 and the common point C. Current will flowthrough relay coil K101 only long enough to charge capacitor C101. Thistime interva is about 2 seconds. Rectifier CR102 is in the circuitmerely to prevent relay coil K101 from "chattering" due to the pulsatingcurrent following through capacitor C101 and rectifier CR101. Thecontact of relay coil K101 (K101/t closes for 2 seconds during thesampling period, and assuming switch S103 to be closed) energizes therelay coil K102 (see description of circuit 13 below). If the cellpotential or voltage circuit 8 is sufficient to energize relay coil K103it remains closed by themagnetic lock-in in that device. This conditionholds sensitroi con tact K103A closed and through the combination ofrelay coil K104 and its contact K104A energizes K102 and holds reinycontact K102A (of circuit 8) open. This turns of! the power supply PS tothe cell and the relay K103 must be mechanically reset for the nexteleetrorefining operation. Rainy coll K104 is inserted in the circuit toreduce the current requirements on the contacts of KtOM. The relay K103is a sensitive device with contacts for a very low current rating suchthat said contacts are in- 4 sufiicient to reliably energize the largecontactor or relay K102. The switch S103 provides the operator with theoption of eliminating a sampling interval without loss 01 timecontinuity.

The following is a description of circuit B of FIGURE 2 as used in apreferred embodiment of this invention:

CIRCUIT B The coil K102 of circuit A energizes two contacts K102A andK1028 of circuit B. K102A is normally closed while K1028 is open so thatpower supply PS can deliver current to the electrochemical cell. Duringthe sampling period, approximately 2 seconds, the power supply is disconnected from the cell and at that time the cell potential is measured.The relay coil K103 and resistor R102 constitute a variable spanvoltmeter whose range is variable from 0 to 1.0 volt. Since the backE.M.F. of the cell during favorable operating conditions is less than0.25 volt, the sensitrol and resistor R102 combination is set at a valueof 0.25 volt. A back E.M.F. greater than 0.25 volt will energize K103and contacts K103A in circuit A during the 2 second sampling period.

Using the device as described above in FIGURE 1 and 2 and in which theanode of the eleetrorefining cell contains a plutonium-gallium alloy theinventors have discovered a method for extending the amount of plutoniumthat can be dissolved anodically before the anode polarization with itsresulting increased back E.M.F. causes the eleetrorefining operation tobe terminated. The following are several experiments conducted showingthe range of additions to the plutonium-gallium anode in preferredembodiment of this invention so as to maximize the amount of plutoniumrecovered using the eleetrorefining system as previously described.

Experiment 1 A one percent by weight gallium-plutonium alloy waselectrorefined by the method described above. Before the eleetrorefiningoperation was commenced, approximately 0.24 percent by weight of ironwas added to the impure anode. The ratio of iron to gallium by weightwas approximately 024. The eleetrorefining of the plutonium wasterminated automatically at an R value (R being the atomic ratio ofgallium to plutonium) of 0.66 which is more than twice the normal ratioof plutonium depletion compared to the theoretical value of 0.33.

Experiment 2 In this experiment the starting gallium concentration was1.95 percent by weight the balance plutonium and 0.52 iron was added tothe anode to give a weight rati of iron to gallium of approximately0.25. This run we terminated automatically at an R value of 0.52.

Experiment 3 The starting gallium concentration was 1.81 weigi percent,the balance being plutonium. Again 0.24 weigi percent of iron was addedto the anode (same amount c iron as Experiment 1), to give aweight-ratio of iron t gallium of 0.133 while the weight ratio of ironto plutt nium was essentially the same as Experiment lt'This ru. wasterminated at an R value of 0.33 thus showing that amount of iron addedtothe anode must be based on the gallium concentration and not on theplutonium concentration.

Experiment 4 Using the same plutonium-gallium composition and conditionsas Experiment 1. 0.007 weight percent iron plus 0015 weight percentnickel, a total of 0.022 percent iron and nickel is added to theplutonium-gallium anode. run was automatically terminated at an R valueof Experiment 3 Under the same conditions as used in Experiment 3 except0.028 weight percent iron and 0.011 weight percent nickel, a total of0.039 percent being introduced into the anode, the electrorefiningprocess was terminated at an R value of 0.39.

Experiment 6 Keeping the same operational procedure as Experiments 1, 3,and 4, 0.041 weight percent iron and 0.025 weight percent nickel, atotal of 0.066 percent was added to the anode. An R value of 0.42 wasobtained before the process was automatically terminated.

The results of these experiments indicate that the adding of a smallamount of iron or iron and nickel to the anode is most beneficial andthat the amount of iron added should be based on the galliumconcentration and not on the plutonium concentration in the alloy whichserves as the anode in the electrorefining process. From the experiments1-3 above, the effective and desired amount of iron added per unitweight of gallium is about 0.24 to 0.27 gram of iron per gram of galliumpresent in the anode alloy. Experiments 4-6 show the amount of iron andnickel that must be present is about 0.25 gram per gram of gallium. Thefollowing elements either alone or in combination with iron also havebeneficial effects in extending the amount of plutonium which is to berecovered using the process and apparatus of this invention:

A. Cobalt, nickel-The total amount of iron plus cobalt and nickel shouldbe about 0.25 gram per gram gallium present in the anode alloy.

B. Ruthenium, rhodium, palladium, osmium, iridium, and

platinum are metallurgically similar to the iron-cobaltnickel additivebut require an underetermined but higher weight ratio of the additive ascompared with the said iron, cobalt, nickel additive.

What is claimed is: 1. An E.M.F. feed-back control circuit which derivesits electrical energy from the back electromotive force of a plutoniumelectrorefining cell reaction between the cathode and anode comprisingin combination a plutonium electrorefining cell, a power supply, and arelay means that can be energized for a period of about 2 seconds at anypreset time interval, said means comprising a motor driven cam switchthat disconnects the power supply and places a calibrated sensitrolvoltmeter across said cell, said voltmeter being preset so at its fullscale reading it will magnetically lock and prevent the said relay meansfrom reconnecting the power supply back across the electrorefining cellcathode-anode circuit.

2. A method for extending the amount of plutonium recovered in theelectrorefining of plutonium in a molten salt electrolyte wherein thesource of plutonium is a plutonium-gallium alloy forming the anode inthe electrolytic cell, comprising the step of initially adding to theanode an amount equal to 24 to 27 weight percent of the galliu m presentin the said anode of at least one element selected from the classconsisting of iron, cobalt, and nickel.

3. The method of claim 2 in which the said element is iron and the saidweight percent of iron that is added to the anode is 25 percent.

4. The method of claim 2 in which the said element is a combination ofiron and nickel.

5. The method of claim 2 in which the said element is a combination ofiron, nickel, and cobalt, and is added to the anode in an amount equalto 25 weight percent of the gallium present.

References Clted UNITED STATES PATENTS 3,098,028 7/1963 Mullins et al.204-l.5 X

REUBEN EPSTEIN, Primary Examiner.

US. Cl. X.R.

